Microwave Oven

Encyclopedia of Food and Culture
COPYRIGHT 2003 The Gale Group Inc.

MICROWAVE OVEN

MICROWAVE OVEN. While experimenting with radar during World War II, Percy Spencer of Raytheon Corporation in Waltham, Massachusetts, discovered the heating properties of microwaves. With a candy bar in his pocket, he leaned in front of the microwave tube and the candy bar promptly melted. This event led to the birth of microwave ovens.

In 1945 Spencer submitted his first patent application for heating food with microwaves. The patent described two parallel magnetrons that heat food that passes by on a conveyor belt. Two years later, William M. Hall and Fritz A. Gross, Spencer's co-workers, applied for a patent for a microwave-heating device enclosed in an oven. This device consisted of two microwave-generating magnetron tubes packed in a metallic box. The oven included a timer and a means of controlling power.

Raytheon's president, Laurence Marshall, was interested in Spencer's patent. A prototype microwave oven was constructed in 1946 costing an estimated $100,000. Marshall was also enthusiastic about the prototype and ordered engineers to develop an oven in which cold sandwiches could be heated. A contest was held to name the new oven—the winner was "Radarange."

Commercial Microwaves

The first commercial Radarange model was a freestanding white-enamel unit operating at 220 volts of electricity and with an internal water-cooling system. The first Raytheon microwave oven was sold to a restaurant in Cleveland, Ohio, in 1947. Subsequent Radaranges incorporated sliding vertical doors. With a price tag of $3,000, sales were mainly limited to restaurants, railroads, cruise ships, and vending-machine companies.

Development of the microwave oven continued during the 1950s. Raytheon dominated the field of commercial microwave ovens and heating applications: It was the only manufacturer of ovens for restaurants and was the principal magnetron manufacturer. Raytheon licensed other companies, such as Hotpoint, Westinghouse, Kelvinator, Whirlpool, and Tappan, to manufacture the ovens. Raytheon furnished power supplies, magnetrons, and basic-oven design data to each company. The Tappan Company began experimenting with a Radarange installed in their lab. Tappan engineers, who
were experts in cooking, teamed up with the Raytheon microwave engineers. In January 1952 the Tappan Company developed the first domestic commercial Radarange. It was powered by a 1,400-to 1,700-watt magnetron that was water cooled and required plumbing connections. The unit was five and a half feet high and weighed 750 pounds.

Domestic Microwaves

The experimental unit developed by Tappan was impractical for domestic use. What was needed was a magnetron requiring less power and a heat dislocation system that could replace the water cooling mechanism. Tappan engineers designed a cabinet with an air-cooled system. Eventually, the magnetron and related components, which had fed microwaves directly into the cavity, were relocated behind the oven. In October 1955, Tappan introduced the first domestic microwave oven for the consumer market. Designed to fit a standard forty-inch range or for built-in use, the unit had a stainless-steel exterior and aluminum oven cavity with a glass shelf. The oven featured two cooking speeds (500 or 800 watts), a browning element, timer, and a recipe-card file drawer. It retailed for $1,295. The unit was marketed as an "electric range." Its advertised advantages were cooking speed, a cool oven, and a unique reheating capability.

General Electric's Hotpoint division, which also had been researching microwave cooking, unveiled its electronic oven the following year. Both the Tappan and Hotpoint oven generated unprecedented enthusiasm and interest in 1956, but sales were dismal. The price was high for the average consumer, and food-processing techniques for the microwave were not well understood. Few food processors took the technology seriously, thus few microwaveable foods were produced.

Breakthroughs

Tappan continued to improve its product. By 1965 Tappan had introduced the first "microwave cooking center," which consisted of a microwave oven mounted above a conventional range. This unit still retailed for well over $1,000. Despite these advances, only ten thousand households in the United States owned microwaves by 1966.

Two events revolutionized the microwave industry. The first was the invention by Keisha Ogura of the New Japan Radio Company—40 percent of which was owned by Raytheon—of a compact, low-cost magnetron. The second was Raytheon's acquisition of Amana Refrigeration, Inc. George Forestner, Amana's president, was a microwave visionary. Amana appliance engineers teamed up with Raytheon experts to develop and design a household Radarange. In August 1967, Amana released its first microwave oven, the Amana RR-1. It operated at 115 volts and sold for $495. The unit was well received. The Amana RR-1 set off a revolution in microwave oven technology, and Amana's success encouraged other appliance manufacturers to produce microwave ovens.

Another important microwave oven manufacturer was Litton, which acquired a small microwave manufacturer called Heat & Eat in 1964. Previously, Litton had manufactured commercial microwave ovens for restaurants. Its newly named Microwave Cooking Products Division in Minneapolis targeted the home market. Litton's Model 500 used 115 volts and was compact. These ovens were installed on TWA planes in 1965, and Litton dominated the restaurant business by 1970.

Microwave Challenges

Despite the initial successes, there were still problems to overcome before the microwave oven would be generally accepted. Manufacturers needed to convince the public that microwave ovens were safe. This fear began with the U.S. Congress's passage of the Radiation Control for Health and Safety Act in 1968. On 4 January 1970, the U.S. Department of Health, Education, and Welfare published the results of microwave oven radiation tests. The tests showed that microwave ovens leaked microwaves. Thus the federal government developed new standards and required changes in the construction of ovens beginning on 6 October 1971. These new regulations required design changes that would result in safer microwave ovens. Public apprehension slowly abated.

Another crucial challenge was convincing food processors to repackage their products. Foods packed in foil blocked microwaves and damaged ovens. Also, frozen foods contained too much water for microwave use. At first, food processors were not interested in working with microwave manufacturers. By the 1970s, however, more than 10 percent of all U.S. homes possessed microwaves, many microwave ovens were in use in vending businesses, and numbers were steadily increasing. Major food processors quickly reversed their direction and invested in microwaveable food products, and specialized microwave cookware was introduced. By 1975 microwave ovens out-sold gas ranges, with sales of over one million units. In the early twenty-first century, the primary use of microwave ovens in the United States was to reheat food.

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microwave oven

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

microwave oven, device that uses microwaves to rapidly cook food. The microwaves cause water molecules in the food to vibrate, producing heat, which is distributed through the food by induction. A special electron tube called a magnetron produces the microwaves. Typical output power for consumer devices ranges from 650 to 1200 watts. To ensure even heating, the magnetron directs its waves at a rotating metal disk with offset vanes, which scatters the waves through the oven cavity; a rotating platform for the food is sometimes used in addition. Power settings may reduce the amount of radiation by cycling a constant-output magnetron on and off for varying lengths of time, or may reduce the level of radiation constantly produced by an inverter magnetron. The magnetron may be supplemented by quartz and halogen bulbs for browning food, which microwaves do poorly.

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microwave

mi·cro·wave
/ ˈmīkrəˌwāv/
•
n.
an electromagnetic wave with a wavelength in the range 0.001–0.3 m, shorter than that of a normal radio wave but longer than those of infrared radiation. Microwaves are used in radar, in communications, and for heating in microwave ovens and in various industrial processes. ∎ short for microwave oven.
•
v. [tr.]
cook (food) in a microwave oven.
DERIVATIVES:mi·cro·wave·a·ble
(also mi·cro·wav·a·ble) adj.

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microwave

microwave Form of electromagnetic radiation having a wavelength between 1mm (0.04in) and 1m (3.3ft) and a frequency range of about 255 to 300,000MHz. Microwaves are used in radar, radio and television broadcasting, high-speed microwave heating and cellular telephones.

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microwave

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

microwave, electromagnetic wave having a frequency range from 1,000 megahertz (MHz) to 300,000 MHz, corresponding to a wavelength range from 300 mm (about 12 in.) to 1 mm (about 0.04 in.). Like light waves, microwaves travel essentially in straight lines. They are used in radar, in communications links spanning moderate distances, and in other applications, such as microwave ovens. The equipment used to generate, process, and transmit microwaves is in many respects different from that used with lower frequency radio waves. See waveguide; magnetron.

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Microwave Oven

How Products Are Made
COPYRIGHT 1996 Gale Research Inc.

Microwave Oven

Background

Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).

Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.

The ability of microwave energy to cook food was discovered in the 1940s by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for the military during World War II. Spencer's experiments revealed that, when confined to a metal enclosure, high-frequency radio waves penetrate and excite certain type of molecules, such as those found in food. Just powerful enough to cook the food, the microwaves are not strong enough to alter its molecular or genetic structure or to make it radioactive.

Raytheon, the company for which Dr. Spencer was conducting this research, patented the technology and soon developed microwave ovens capable of cooking large quantities of food. Because manufacturing costs rendered them too expensive for most consumers, these early ovens were used primarily by hospitals and hotels that could more easily afford the $3,000 investment they represented. By the late 1970s, however, many companies had developed microwave ovens for home use, and the cost had begun to come down. Today, microwaves are a standard household appliance, available in a broad range of designs and with a host of convenient features: rotating plates for more consistent cooking; digital timers; autoprogramming capabilities; and adjustable levels of cooking power that enable defrosting, browning, and warming, among other functions.

Design

The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the
door frame has a small window to enable the cook to view the food while it is cooking.

Near the top of the steel oven cavity is a magnetron—an electronic tube that produces high-frequency microwave oscillations—which generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.

Raw Materials

The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables.

The ManufacturingProcess

Oven cavity and door manufacture

1 The process of manufacturing a microwave oven starts with the cavity and the door. First, the frame is formed using automatic metal-forming presses that make about 12 to 15 parts per minute. The frame is then rinsed in alkaline cleaner to get rid of any dirt or oil and further rinsed with water to get rid of the alkaline solution.

2 Next, each part is treated with zinc phosphate, which prepares it for electro-deposition. Electro-deposition consists of immersing the parts in a paint tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5 mils thick. The parts are then moved through a paint bake operation where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius) for 20 minutes.

3 After the door has been painted, a perforated metal plate is attached to its window aperture. The plate reflects microwaves but allows light to enter the cavity (the door will not be attached to the cavity until later, when the chassis is assembled).

The magnetron tube subassembly

4 The magnetron tube assembly consists of a cathode cylinder, a filament heater, a metal anode, and an antenna. The filament is attached to the cathode, and the cathode is enclosed in the anode cylinder; this cell will provide the electricity that will help to generate the microwaves. Metal cooling fins are welded to the anode cylinder, and a powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated. A metal strap holds the complete assembly together. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.

5 An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. The waveguide is connected to the magnetron on top of the protruding antenna, while a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. Finally, a plastic fan is attached to the motor, where it will draw air from outside the oven and direct it towards the vanes. This completes the magnetron subassembly.

Main chassis assembly

6 The chassis assembly work is performed on a pallet—a work-holding device used in conjunction with other tools—located at the station. First, the main chassis is placed on the pallet, and the cavity is screwed on to the chassis. Next, the door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and the main chassis.

7 The circuit that produces the voltage required to operate the magnetron tube consists of a large transformer, an oil-based capacitor, and a high voltage rectifier. All of these components are mounted directly on the chassis, close to the magnetron tube.

Stirrer fan

8 The stirrer fan used to circulate the microwaves is mounted on top of the cavity. Some manufacturers use a pulley to drive the fan from the magnetron blower motor; others use a separate stirrer motor attached directly to the fan. Once the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce arcing and damage the magnetron.

Control switches, relays, and motors

9 The cook switch provides power to the transformer by energizing a relay and a timer. The relay is mounted close to the power transformer, while the timer is mounted on the control board. The defrost switch works like the cook switch, activating a motor and timer to operate the defrost cycle. Also mounted on the control board are a timer bell that rings when the cooking cycle is complete and a light switch that allows viewing of the cavity. A number of interlocking switches are mounted near the top and bottom of the door area. The interlocking switches are sometimes grouped together with a safety switch that monitors the other switches and provides protection if the door accidently opens during oven operation.

Front panel

10 A front panel that allows the operator to select the various settings and features available for cooking is attached to the chassis. Behind the front panel, the control circuit board is attached. The board, which controls the various programmed operations in their proper sequence when the switches are pushed on the front panel, is connected to the various components and the front panel by means of plug-in sockets and cables.

Making and assembling the case

11 The outer case of the microwave is made of metal and is assembled on a roll former. The case is slipped onto the preassembled microwave oven and bolted to the main chassis.

Testing and packaging the oven

12 The power cords and dial knobs are now attached to the oven, and it is sent for automatic testing. Most manufacturers run the oven from 50-100 hours continuously as part of the testing process. After testing is complete, a palletizer robot records the model and serial data of the oven for inventory purposes, and the oven is sent for packaging. This completes the manufacturing process.

Quality Control

Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.

In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.

The Future

Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.

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Microwave Oven

Microwave Oven

A microwave oven is an appliance that cooks or warms up food using microwaves produced by an electronic vacuum tube called a magnetron. The magnetron converts electricity to electromagnetic radiation (the microwaves), which is made up of waves of electric and magnetic energy. Once generated by the magnetron, the microwaves travel through a metal enclosure called a waveguide to a stirrer fan, which distributes the microwaves into the cooking cavity. Inside the cooking area, the microwaves are absorbed by the food, which is cooked or heated in a few minutes or seconds.

Efficient cooking with microwaves

Microwaves can be used in cooking because of certain characteristics. They pass through many types of glass, plastic, ceramic, and paper. They are reflected by metal, and they are absorbed by food. In microwave cooking, the microwaves act on the water molecules in the food, causing them to vibrate. The vibration causes friction, which generates heat, thereby cooking the food. Microwaves cook food more efficiently than conventional ovens because they act only on the food, unlike regular ovens
which heat the oven walls and the air around the food. And since microwaves change to heat once they are absorbed by the food, the food does not get contaminated with the electromagnetic radiation.

First microwaved foods

The ability of microwave energy to cook food was discovered in 1945 by an engineer in the Raytheon Company. Percy L. Spencer (1894–1979) was inspecting a magnetron at work. (During World War II [1939–45], the magnetron tube had been used in radar systems.) Spencer discovered that a chocolate bar in his pocket had melted even though he had not felt any heat from the magnetron. Next, Spencer exposed some popcorn kernels to the magnetron, and, sure enough, the kernels popped.

Spencer discovered that, when confined to a metal enclosure, microwaves produced by the magnetron tube excite certain molecules, such as water found in food, causing the food to increase in temperature and eventually get cooked. The first microwave oven, called Radarange, developed by Spencer and the Raytheon Company, was about the size of a refrigerator. It was used in commercial food preparations, and cost about $3,000. In 1955, slightly smaller versions were sold as home appliances, but they were still expensive, and people were not sure they wanted an appliance that they thought emitted "radar waves."

Fast food, fast bucks

In 1967, Amana, a Raytheon division, introduced the first countertop models at a cost of $495. By the mid-1970s, microwave ovens were outselling gas ranges. Fears about the potential dangers of microwaves had dissipated, and different companies were manufacturing microwave ovens for home use. Since then, microwave ovens have become a popular kitchen fixture. About 95 percent of American households own a microwave oven. Related industries, such as microwavable foods and utensils designed to be used especially for microwave cooking, have also grown rapidly.

Design

A microwave oven has a front panel that allows the user to program the oven. The panel shows the cooking time, the power level, the time of day, and other information. The door has a window with a perforated (having small holes) metal shield that enables the user to view the food
while it is cooking. Microwaves cannot go through the holes to cause any harm because the holes are smaller than the wavelengths of the microwaves. As a result, microwaves simply bounce off the shield to act on the food. An oven light is also included.

The various electronic motors, relays, and control circuits (the routes through which electricity flows) are situated on the outer casing, to which the oven cavity is bolted, or fastened. The cooking cavity may contain a round, glass turntable on which the food dish is placed. The turntable rotates the food dish for even cooking. Instead of a turntable, the cooking cavity may just have a stationary (nonmoving) glass dish, which fits the bottom of the cavity.

Near the top of the cooking cavity is the magnetron tube, which produces the microwaves. The microwaves are funneled through a metal tube called a waveguide and into a stirrer fan. The fan distributes the microwaves evenly within the oven. Manufacturers use different methods to circulate the microwaves to achieve even cooking. Some use dual stirrer
fans located on opposite walls. Others, in addition to the stirrer fan on top of the cavity, use entry ports (openings) at the bottom of the cavity so that microwaves enter through both the top and bottom.

Raw Materials

The outer covering of a microwave oven is usually a one-piece sheet metal enclosure. The door and the panels of the cooking cavity are made of stainless steel or galvanized steel (steel that has been coated with a thin layer of zinc to protect against rusting). A coating of acrylic enamel is also applied to the panels. The cooking surface is generally made of ceramic or glass. The cooking cavity is usually painted with a light color so that it is easy to see from outside the oven.

Between the cooking cavity and the oven walls, electromechanical components and controls consist of timer motors, switches, and relays. Also inside these walls are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the different components are made up of metal and plastic parts, including gears, pulleys, belts, nuts, screws, washers, and cables.

The Manufacturing Process

Most parts of the modern microwave oven are mounted in the area behind the front panel, between the sidewall of the cooking cavity and the chassis sidewall. Manufacturers aim to make ovens that are not too bulky. Therefore, they make sure the parts are efficiently situated within the available space, allowing enough room for the great amount of heat generated by the magnetron.

MAKING WAVES

How does a magnetron tube make microwaves? When heat is applied to the cathode (also called the filament), which is at the center of the magnetron, the cathode produces electrons, or negative electricity. The electrons are attracted to the anode (positive electrode) cylinder that surrounds the cathode. The electrons start traveling straight toward the walls of the anode cylinder. However, two magnets on top and below the cathode-anode structure cause the electrons to spin, thereby generating powerful microwaves. These microwaves go through an antenna above the anode, pass through a metal enclosure called a waveguide, and are distributed into the cooking cavity by a stirrer fan.

The cooking cavity and door

1 The manufacture of a microwave oven starts with the cooking cavity and the door. The cavity panels are formed using automatic metal-forming presses that can make twelve to fifteen parts per minute. In metal-forming, pressure is applied by the press on metal, changing its shape to form the cavity parts. The panels are rinsed in alkaline cleaner to
remove dirt or oil. Then, they are rinsed with water to remove the alkaline solution.

2 The cavity panels and door are treated with zinc phosphate to prepare them for electro-deposition, during which the pieces are immersed in a paint tank and electric current is used to apply the paint. Then, the pieces are moved through a paint-bake operation to set the paint coating at 300 degrees Fahrenheit (149 degrees Celsius) for twenty minutes.

3 After the door is painted, a perforated metal shield is attached to the window opening. The shield reflects microwaves but allows light to enter the cavity. The door will not be attached to the cavity until after the chassis, or main framework, is assembled.

The magnetron tube subassembly

4 The magnetron tube consists of a filament heater, a metal anode, two magnets, and an antenna. The filament, which functions as the cathode, is enclosed in a ring-shaped metal anode. Then the cathode and the anode are sandwiched between two magnets. These components make up the assembly that will generate the microwaves. A metal strap holds the complete assembly together. Metal cooling fins, which are welded to the anode cylinder, help get rid of the heat produced by the magnetron. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.

5 An antenna enclosed in a glass tube is connected to the anode cylinder. Then, the air within the magnetron tube is pumped out to create a vacuum. The antenna is connected to the waveguide, the hollow metal enclosure through which microwaves are transported from the magnetron tube to the cooking cavity. A blower motor is attached to the magnetron, and then a plastic fan is attached to the blower motor to provide cool air to the magnetron to prevent overheating.

Chassis assembly

6 The chassis is the major framework onto which the various oven components will be attached. The chassis assembly work is done on a pallet, a work-holding device used with other tools. The chassis is placed on the pallet, and the cooking cavity is screwed on to the chassis. The door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and chassis.

7 The electric circuit of the oven consists of the transformer (which generates the high-voltage, or powerful electricity, required to operate the magnetron tube), an oil-based capacitor (which receives and stores electric current), and a rectifier (which changes the alternating current from the transformer into the direct current needed by the magnetron). All of these are mounted directly on the chassis, close to the magnetron tube.

Stirrer fan

8 The stirrer fan that circulates the microwaves is mounted on top of the cooking cavity. Some manufacturers use a pulley to operate the fan from the magnetron blower motor. Others use a separate stirrer motor attached directly to the fan. After the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce sparks and damage the magnetron tube.

Control switches, relays, and motors

9 The cook switch provides power to the transformer by energizing a cook relay and a timer. The cook relay, also called a power relay, allows the control board to turn the microwave source off and on. The relay is mounted close to the power transformer, while the timer is mounted on the control board. Also mounted on the control board are a light switch for viewing the cooking cavity and a timer bell that rings when the cooking cycle is complete.

SUPERHEATING HAZARD

People have reported hot liquids exploding inside microwave ovens or being burned when they first introduced a teaspoon, sugar, or teabag to a hot liquid right out of the oven. These accidents are caused by superheating. If a liquid does not boil (steam bubbles do not form during heating), it tends to heat past the boiling point of 212 degrees Fahrenheit (100 degrees Celsius). Normally, when liquid is heated over the stovetop, small bubbles form on the bottom of the heating container even before the boiling point is reached. The escaping bubbles ensure that the liquid boils. With microwave cooking, the liquid typically gets heated uniformly and no bubbles form. Objects introduced to the superheated liquid initiate the boiling process, causing it to explode.

A number of interlocking switches, also called safety switches, are mounted near the top and bottom of the door area. These switches prevent
the production of microwaves when the door accidentally opens during cooking.

Front panel

10 Also attached to the chassis is the front panel, which allows the user to select the various settings and features available for cooking. Mounted behind the front panel is the control board, which is an electronic board that controls the various programmed operations when the switches are pushed on the front panel. This board is connected to the front panel and other components by means of plug-in sockets and cables.

Making and assembling the case

11 The outer case of the microwave oven is made from sheet metal, made by pressing steel between rollers. The sheet metal is formed into a wraparound case and fitted with a bottom
plate, also made of steel. The finished case is slipped onto the preassembled oven and bolted to the main chassis. Finally, power cords and dial knobs, if any, are attached to the oven.

Testing and packaging the oven

12 Most manufacturers run the oven from fifty to one hundred hours continuously as part of the testing process. After testing is complete, a robot palletizer records the model and serial data of the oven for inventory purposes. A robot palletizer, which is fitted with a gripper unit, is also used in many industries for loading finished products onto pallets, or platforms. Finally, the microwave oven is sent to the packaging area.

Quality Control

Strict quality control throughout the manufacturing process is very important. Radiation that is emitted by the microwave oven can burn anyone exposed at high levels for prolonged periods. A computer-controlled scanner is used to measure radiation leaks around the door, window, and back of the oven. Other scanners check antenna radiation, as well as the seating of the magnetron tube. Each scanner operation passes on information to the next operation so that problems can be corrected.

The Code of Federal Regulations, revised as of April 1, 2001, limits the amount of radiation that can leak from a microwave oven to "1 milliwatt per
square centimeter at any point 5 centimeters or more from the external surface of the oven, measured prior to acquisition by a purchaser, and, thereafter, 5 milliwatts per square centimeter at any such point." The regulation also requires all ovens to have a minimum of two safety interlocks to prevent the production of microwaves if the door accidentally opens during cooking.

The Future

Manufacturers continue to develop various models and new applications to appeal to new users and repeat consumers. Like other industries that are taking advantage of the popularity of the Internet, the microwave industry has introduced the so-called Internet microwave oven. The oven can be connected to the Internet to access recipes and other cooking information for automatic food preparation. Several companies plan to market their products in the near future.

Other new developments include a microwave oven with special brackets that allow installation anywhere in the kitchen, including inside wall cabinets and over islands (the freestanding areas usually found in the middle
of the kitchen). Some models have the control panel located behind the oven door, thus giving the oven an overall sleek appearance. A trendy model that appeals to many of today's consumers is patterned after a popular computer monitor that comes in colors of red, orange, green, and blue.

DID YOU KNOW?

• Contrary to a popular myth, a microwave oven does not cook from the inside out. If the food is small, the microwaves penetrate it, cooking it all at once. If the food is large, the outside gets cooked by microwaves, while the inside is cooked by the heat transferred from the hot exterior portion of the food.

• Manufacturers caution users not to put metal utensils or aluminum foil inside the microwave oven. These objects, like the metal walls of the oven, will reflect microwaves. When the powerful microwaves act on the walls and the metal objects, the air between them becomes charged with electricity, thereby causing sparks. These sparks can damage the cooking cavity.

In 2000 two Massachusetts inventors received a patent for a microwave oven that can be installed in a car's glove compartment. The oven would be powered by the car's battery and the controls would be mounted on the dashboard. The oven comes with a bin that could store things when the oven is not being used. The oven would only work when the bin is not in place.

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